By using the --with-mysql[=DIR]
configuration option you enable PHP to access MySQL
databases.

In PHP 4, the option --with-mysql is
enabled by default. To disable this default behavior, you may use the
--without-mysql configure option.
Also in PHP 4, if you enable MySQL without specifying the path to the
MySQL install DIR, PHP will use the bundled MySQL client
libraries. In Windows, there is no DLL, it's simply built into PHP 4.
Users who run other applications that use MySQL (for example,
auth-mysql) should not use the bundled library, but rather specify the
path to MySQL's install directory, like so:
--with-mysql=/path/to/mysql.
This will force PHP to use the client libraries
installed by MySQL, thus avoiding any conflicts.

In PHP 5, MySQL is no longer enabled by default, nor is the MySQL library
bundled with PHP. Read this
FAQ for details on why. Because of this, Windows users will need
to enable php_mysql.dll inside of php.ini and either
copy libmysql.dll into the Windows system directory or
make it available to the PATH. For compiling, simply use
--with-mysql=[DIR] where
[DIR] points to your MySQL installation directory.

This MySQL extension doesn't support full functionality of MySQL versions greater than
4.1.0. For that, use MySQLi.

If you would like to install the mysql extension along with the mysqli extension
you have to use the same client library to avoid any conflicts.

Warning

Crashes and startup problems of PHP may be encountered
when loading this extension in conjunction with the recode extension.
See the recode extension for more
information.

Note:
If you need charsets other than latin (default), you
have to install external (not bundled) libmysql with compiled charset
support.

The default TCP port number to use when connecting to
the database server if no other port is specified. If
no default is specified, the port will be obtained
from the MYSQL_TCP_PORT environment
variable, the mysql-tcp entry in
/etc/services or the compile-time
MYSQL_PORT constant, in that order. Win32
will only use the MYSQL_PORT constant.